Brief description of the key process of manufacture of double sided printed board

Double-sided PCB is a printed board with conductive graphics on both sides. It is usually made of epoxy glass cloth covered with copper foil board. It is mainly used in communication electronic equipment, advanced instruments and electronic computers and other equipment with high performance requirements.

1. CNC drilling

Due to the development of SMT (surface assembly technology), the plating holes on printed boards are no longer used to insert electronic components, only for general use. In order to increase assembly density, the holes are becoming smaller and smaller, so machining often uses a new generation of CNC drilling machines that can drill small holes. The CNC drilling machine has a drilling rate of 110 000 ~ 150 000 R /min and can drill holes of 0.1 ~ 0.5mm in diameter. When the bit is broken, it will automatically stop and alarm, automatically replace the bit and measure the diameter of the bit, and automatically control the constant distance and drilling depth between the bit and the cover plate, so that not only through holes can be drilled, but also blind holes can be drilled.

2. Plating process

Coated holes (PTH), also known as metallized holes. It is the whole hole wall coated with metal, so that the two sides of the double-sided printed board or the inner and outer layers of the multi-layer printed board between the conductive graphics to achieve electrical communication. Metallized hole is the most critical link in the production process of double panel and multilayer board, which is related to the internal quality of multilayer board. Electroless copper plating is traditionally used to deposit a thin layer of copper on the wall of the hole, and then the copper is plated to a specified thickness. Some new pore-coating processes, such as direct plating without electroless copper plating, have been developed.

Step 3: Imaging

Screen printing imaging can also be used to make double panels. It is low cost and suitable for mass production, but difficult to make fine wire and fine spacing double panels below 0.2mm. In addition to the use of screen printing, polyvinyl alcohol/dichromate type liquid photoresist was widely used in the 1950s and 1960s. In 1968, DuPont Company launched dry film photoresist (dry film for short), in the 1970s and 1980s, dry film imaging technology has become the leading technology of double panel imaging. In recent years, due to the development of new liquid photoresist, its resolution is higher than that of dry film, and the coating equipment of liquid photoresist has been able to achieve continuous large-scale production, and the cost is cheaper than dry film, so liquid photoresist has a tendency to be used in large quantities again.

The process flow of dry film imaging includes: pre-coating treatment → coating → exposure → development → repairing plate → etching or electroplating → removing film.

Before the treatment of abrasive nylon roller brush board machine or pumice stone brush board machine for the brush board, after the treatment of the plate with the film machine for double-sided continuous film. The main process parameters of the film are: temperature, pressure and speed.

After electroplating or etching, the dry film should be removed. To remove the film generally use 4% ~ 5% sodium hydroxide solution, in the temperature of 40 ~ 60℃ in the spray film machine. After removing the film, wash thoroughly with water before entering the next process.

4. Tin-lead alloy plating

In the production of double panels by graphic electroplating etching, the tin lead alloy has two functions: one is to act as the corrosion protection layer during etching; The second is the solderable coating as the finished plate. As a weldable coating, the proportion of tin and lead in the coating and the microstructure of the alloy are required. But in SMOBC process, tin - lead electroplating is only used as etching protection layer. In this case, the requirement for the proportion of tin and lead is not high, so the tin content in the range of 58% to 68% can meet the requirements. The plating solution and process conditions must be strictly controlled for the plating of tin lead alloy. The plating thickness of tin lead alloy shall be more than 8μm on the plate surface and no less than 2.5μm on the hole wall.

5. Etching

In the manufacture of double panels by graphic plating etching with tin lead alloy as the resist layer, neither acid copper chloride etching solution nor ferric chloride etching solution can be used because they also corrode tin lead alloy. The etching fluids that can be used are alkaline copper chloride etching solution, hydrogen peroxide sulfate etching solution, ammonium persulfate etching solution, etc. One of the most used is alkaline copper chloride etching solution.

In the etching process, the phenomenon of "side erosion" and coating widening affects the etching quality.

(1) Lateral erosion. Lateral erosion is the recessed or hollowed out phenomenon of wire edge caused by etching. The degree of lateral erosion is related to etching fluid, equipment and technological conditions. The smaller the lateral erosion, the better. The use of thin copper foil can reduce the side corrosion value and is conducive to the manufacture of fine wire pattern.

(2) Widened coating. The widening of the coating is due to the plating thickening so that the width of one side of the wire exceeds the value of the width of the production plate. As a result of the side erosion and coating widening, the wire pattern produces a coating edge. Coating edge is the sum of plating width and side erosion, it not only affects the accuracy of the graphic, but also easily breaks and falls, resulting in short circuit. The edge of the coating can be eliminated after hot melting.

The etching coefficient is the ratio of etching depth (wire thickness) to the amount of side erosion. When making fine wire, the etching coefficient can be improved by using vertical jet etching or adding side protector.

Step 6 Gilt

Gold coating has excellent electrical conductivity, small contact resistance and stability, good wear resistance, is the best coating material for printed board plug. It also has excellent chemical stability and weldability, and is also used as corrosion resistant, weldable and protective coating on surface assembly printed circuit boards. Because the price of gold is very expensive, generally in order to save costs, as far as possible thinner plating, especially the full plate gold-plated printed board, generally using flash gold or chemical gold plating, commonly known as plating "water gold", its thickness is less than 0.1μm, only about 0.05 ~ 0.1μm. However, the gold coating of the plug part should be thicker, and the thickness should be 0.5-2.5μm according to different requirements. If the copper is directly gilded, because the gold coating is thin, the coating has more pinholes, in the long-term use or storage process, through the pinhole copper will be corroded; In addition, the diffusion of metal compounds between copper and gold is easy to make the solder joint brittle, resulting in unreliable welding. Therefore, before gold plating are required to use nickel plating layer. The thickness of nickel plating layer is generally controlled within 5 ~ 7μm. The process of nickel plating on the plug is as follows: stick protective tape → tin lead → water washing → micro etching or scrub → water washing → activation → water washing → nickel plating → water washing → activation → water washing → gold plating → water washing → drying → strip off → inspection.

7. Hot melt and hot air leveling

(1) Hot melt

The hot melting process of PCB board is: the printed board coated with tin lead alloy is heated to above the melting point temperature of tin lead alloy, so that tin lead and matrix metal copper form metal compounds, at the same time, the tin lead coating becomes dense, bright, no pinhole, and improve the corrosion resistance and weldability of the coating. Glycerine hot melt and infrared hot melt are commonly used in hot melt.

(2) Hot air leveling

Hot air leveling, also known as tin spraying, is the main process of SMOBC process. The process is: the printed board has been coated with solder resistance after hot air smoothing the flux, then immersed in the molten solder tank, and then through the two air knife, the hot compressed air between the printed board surface and the hole to blow off the excess solder, get a bright, uniform, smooth solder coating. The typical process flow of hot air leveling is:

Bare copper plate → gold-plated plug paste protective tape → pretreatment → coated flux → hot air leveling → cleaning → remove tape → inspection.

Pre-treatment includes steps such as oil removal, cleaning, weak corrosion, washing and drying to obtain a clean and slightly coarse surface free of oil and oxide.

(3) Main process parameters of hot air leveling

The main process parameters of hot air leveling include solder temperature, dip welding time, Angle between air knife and printed board, air knife gap, hot air temperature, pressure and flow rate, preheating time and temperature, circuit board lifting speed, etc. The solder tank temperature is generally controlled at 230 ~ 235℃, the air knife temperature is controlled above 176℃, the dipping time is controlled at 5 ~ 8s, and the coating thickness is controlled at 6 ~ 10μm.